Electric dipole fields show an increasing contribution to ultimate intrinsic signal-to-noise ratio (“SNR” or “UISNR”) at high fields. (See, e.g., References 11 and 12). Substantial SNR boosts are shown in a body sized phantom at magnetic fields of, for example, 7 Tesla (“7 T”), by using a combination of loop and electric dipole elements. (See, e.g., Reference 13). A mixture of element types can also improve the performance of parallel transmit array designs. (See, e.g., Reference 14). However, as most parallel transmit systems are currently limited to eight channels design options are limited.
At fields of, for example, 7 T and higher, electric dipole antennas can be applied as transmit and receive coils for magnetic resonance (“MR”) imaging. (See, e.g., References 1, 2 and 3). There are challenges in applying electric dipole antenna designs for MR imaging. A self-resonant half wavelength dipole antenna for about 300 MHz (e.g., the 7 T operating frequency) can be between, for example, 35 and 50 cm long, depending on its proximity to tissue, which is inconveniently long for many applications. Placing a slab of high dielectric material between the dipole and the body can shorten the length, but such blocks of material are bulky (see e.g., Reference 4), and high specific absorption rate (“SAR”) can be generated at the edges of the block. (See, e.g., Reference 4).
The inclusion of lumped element inductors, or meanders, along the length of the dipole antenna can shorten it, but often at the cost of increased losses. Typical measures used to decouple arrays of surface coil loops, such as overlap or decoupling circuitry, may not be applicable to dipole antennas, limiting the density of dipole antenna placement with current designs. To achieve improved performance at 7 T, a combination of magnetic loop and electric dipole elements can be desired (see, e.g., Reference 5 and 13), doubling the number of elements needed to provide sample coverage and diverse antenna properties.
A loop element with a non-uniform current distribution behaves as both a loop antenna and an electric dipole antenna1. At 7 Tesla in a body sized phantom this “loopole” element can achieve either higher central B1+ or higher SNR as compared to a traditional balanced current loop depending on the orientation of the excitation port. (See e.g., References 18 and 19). With increasing current imbalance the loopole performs more like an electric dipole antenna, which has been shown to be favorable for central SNR in body sized objects at 7 T. (See e.g., Reference 20). Nevertheless the loopole coils can still be fabricated like conventional surface coil loops, for example, tuned with trimmer capacitors, decoupled through overlapping and tiled to make dense transceiver arrays. A loopole coil also exhibits reduced loading sensitivity compared to an electric dipole, where proximity to the conductive object results in significant tune and match variation. (See e.g., Reference 21). These properties make the loopole a desirable building block in practical transceiver coil arrays to image centrally located body regions.
Achieving adequate flip angle in the spine at 7 T has been a challenge due to its central location, limited available RF power and SAR constraints. In this work we designed a four element loopole array consisting of an anterior and posterior shell with two overlapped loopole elements in each half for human spine imaging at 7 T. In each pair of elements one is used to transmit and both are used to receive. We performed bench measurements, simulations and MR experiments comparing the proposed design with a variety of previously described 7 T spine arrays. (See e.g., References 22 and 23).
Thus, it may be beneficial to provide exemplary circular dipole and surface coil loop structures, which can overcome at least some of the deficiencies described herein above.